Improvements on KeyEventTest.

These improvements will make it easier to change KeyEvent
constructors:

- Replaced JUnit assertions by Truth's.
- Add a message explaining which field assertion failes.
- Use proper APIs to assert collections.
- Replaced some assertions to use expect.

Test: atest KeyEventTest

Bug: 266851112
Bug: 268103680

Change-Id: I79a35d8dd3a0bc7eca0650c4d2b42613c94e9d8e